Internally stiffened composite panels and methods for their manufacture

ABSTRACT

Internally stiffened composite panels and associated manufacturing methods. In one embodiment, an internally stiffened composite panel includes a first face sheet, a second face sheet offset from the first face sheet, and at least one core portion sandwiched between the first and second face sheets. In one aspect of this embodiment, the composite panel further includes a stiffener disposed between the first and second face sheets adjacent to the core portion. The stiffener can have a first attachment portion attached to the first face sheet, a second attachment portion attached to the second face sheet, and an intermediate portion extending between the first and second attachment portions. In another aspect of this embodiment, one or both of the first and second attachment portions is configured to engage a fastener, such as a blind fastener, for attaching a part to the composite panel.

TECHNICAL FIELD

The following disclosure relates generally to composite panels and, moreparticularly, to composite panels having internal stiffeners that canengage fasteners for attaching parts to the composite panels.

BACKGROUND

Composite materials are often used in the construction of aerospace andother structures because of their high strength-to-weight ratios. Manyaircraft, for example, utilize composite materials for both internalairframe components and external skin panels. Composite materials areoften used in the form of a panel that can include a core materialsandwiched between two opposing face sheets. The face sheets can includeone or more plies of a composite fabric material, and the core caninclude a lightweight material such as a foam or honeycomb material. Theresulting combination provides a lightweight panel having relativelyhigh strength and stiffness. The panel can be fastened or otherwiseattached to frames or other parts as required to complete a particularassembly.

FIGS. 1A-C are cross-sectional views of portions of composite panelassemblies 100 a-c having composite panels 101 a-c configured inaccordance with the prior art. Referring to FIG. 1A, the prior artcomposite panel 101 a includes a first face sheet 102 a, a second facesheet 104 a offset from the first face sheet 102 a, and a core 106 asandwiched between and bonded to the first and second face sheets 102 aand 104 a. A fastener 120 a extends through the composite panel 101 aand attaches a part 110 to the composite panel 101 a.

One feature of the prior art composite panel 101 a is that the fastener120 a extends through holes in both the first and second face sheets 102a and 104 a. A disadvantage of this feature is that water or otherfluids can leak through the fastener hole if one side of the compositepanel 101 a is exposed to these fluids. This shortcoming makes thecomposite panel 101 a a poor choice in those applications where aleak-proof panel is desired. In addition, fluids leaking into thecomposite panel 101 a can compromise the structural integrity of thecore 106 a and/or the bond between the core 106 a and the first andsecond face sheets 102 a and 104 a.

Another feature of the composite panel 101 a is that it has relativelylow compression strength through the panel thickness because the core106 a has a tendency to crush under compression load. A disadvantage ofthis feature is that the fastener 120 a cannot develop sufficientpre-load to adequately clamp the part 110 to the panel 101 a. Toovercome this problem, potting (not shown) can be injected into thecomposite panel 101 a and cured to provide compression strength aroundthe fastener 120 a, preventing the core 106 a from crushing underfastener pre-load. One downside of this solution, however, is thatadditional manufacturing effort is required to install the potting.Further, while this solution may reduce the tendency of the core 106 ato crush, it does not prevent fluid leakage.

Referring next to FIG. 1B, the prior art composite panel 101 b includesa first face sheet 102 b, a second face sheet 104 b, and core portions106 b sandwiched between the first and second face sheets 102 b and 104b. The core portions 106 b taper inwardly toward a coreless region 103 bwhere the second face sheet 104 b is bonded directly to the first facesheet 102 b. A fastener 120 b extends through the first and second facesheets 102 b and 104 b in the coreless region 103 b and attaches thepart 110 to the composite panel 101 b.

One feature of the prior art composite panel 101 b is the lack of panelthickness in the coreless region 103 b. A disadvantage of this featureis that the ability of the composite panel 101 b to withstand largebending moments is substantially reduced. Another disadvantage of thecoreless region 103 b is that tapering the edges of the core portions106 b to form this region requires additional manufacturing expense. Afurther disadvantage is the fluid leakage problem described above withreference to FIG. 1A.

Referring now to FIG. 1C, the prior art composite panel 101 c includes afirst face sheet 102 c, a second face sheet 104 c, and core portions 106c. Potting 107 has been injected into the space between the coreportions 106 c and recessed to provide room for a blind fastener 120 c.The blind fastener 120 c extends through the part 110 and the secondface sheet 104 c and attaches the part 110 to the composite panel 101 c.The blind fastener 120 c is referred to as a “blind fastener” because itcan be fully installed through the second face sheet 104 c with accessfrom only one side of the composite panel 101 c.

One feature of the prior art composite panel 101 c is that the blindfastener 120 c extends only through the second face sheet 104 c. Whilethis feature may prevent the fluid leakage problems described above, ithas the disadvantage of providing a relatively weak attachment for thepart 110. Applying significant loads to the part 110 can cause thesecond face sheet 104 c to deflect and pull away from the potting 107and/or the core portions 106 c in the region adjacent to the blindfastener 120 c. Another disadvantage of this approach is that additionalmanufacturing effort is required to install and recess the potting 107.

SUMMARY

Embodiments of the present invention are directed to composite panelassemblies, such as composite panel assemblies having internallystiffened composite panels. In one embodiment, an internally stiffenedcomposite panel includes a first face sheet, a second face sheet offsetfrom the first face sheet, and at least one core portion sandwichedbetween the first and second face sheets. The at least one core portioncan have a first core face and a second core face opposite the firstcore face. The first core face can be at least partially bonded to thefirst face sheet, and the second core face can be at least partiallybonded to the second face sheet. In one aspect of this embodiment, theinternally stiffened composite panel further includes a stiffenerdisposed between the first and second face sheets adjacent to the atleast one core portion. The stiffener can have a first attachmentportion attached to the first face sheet, a second attachment portionattached to the second face sheet, and an intermediate portion extendingbetween the first and second attachment portions.

In another embodiment, a composite panel assembly includes theinternally stiffened composite panel described above and furtherincludes a part, such as a frame member, fixedly attached to thecomposite panel. In one aspect of this embodiment, the part ispositioned adjacent to the composite panel and proximate to thestiffener. In another aspect of this embodiment, at least one fastenerextends through the part and one of the first and second face sheets toengage the stiffener and fixedly attach the part to the composite panel.

In a further embodiment, a method for manufacturing a composite panelassembly includes providing a first face sheet and positioning anattachment portion of a stiffener at least proximate to the first facesheet. In one aspect of this embodiment, the method further includespositioning at least one core portion adjacent to the first face sheetand at least proximate to the stiffener. In another aspect of thisembodiment, the method also includes positioning a second face sheetadjacent to the at least one core portion and the stiffener.

In another embodiment of the foregoing method, the attachment portion ofthe stiffener is a first attachment portion, and the stiffener furtherincludes a second attachment portion offset from the first attachmentportion by an intermediate portion. In one aspect of this embodiment,the method further includes positioning the second attachment portion ofthe stiffener at least proximate to the second face sheet. In anotheraspect of this embodiment, the method further includes positioning afirst adhesive layer between a first core face and the first face sheet,and positioning a second adhesive layer between a second core face andthe second face sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-C are cross-sectional views of portions of composite panelassemblies having composite panels configured in accordance with theprior art.

FIG. 2 is a cross-sectional view of a portion of a composite panelassembly having an internally stiffened composite panel configured inaccordance with an embodiment of the invention.

FIGS. 3A-D are partially exploded isometric views illustrating amanufacturing sequence for a composite panel configured in accordancewith an embodiment of the invention.

FIG. 4 is a partially cut-away enlarged isometric view of a portion ofthe composite panel of FIG. 3D.

FIGS. 5A-G are cross-sectional views of composite panel assemblieshaving composite panels configured in accordance with other embodimentsof the invention.

FIG. 6 is an isometric view of an aircraft having a composite panelconfigured in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

The following disclosure describes composite panels such as internallystiffened composite panels usable for applications such as aircraftstructural components. Certain specific details are set forth in thefollowing description and in FIGS. 2-6 to provide a thoroughunderstanding of various embodiments of the invention. Those of ordinaryskill in the relevant art will understand, however, that the inventionmay have additional embodiments that may be practiced without several ofthe details described below. In addition, well-known structures andsystems often associated with composite materials and composite panelmanufacturing have not been shown or described in detail here to avoidunnecessarily obscuring the description of the various embodiments ofthe invention.

In the drawings, identical reference numbers identify identical orgenerally similar elements. To facilitate the discussion of anyparticular element, the most significant digit or digits of anyreference number refers to the figure in which that element is firstintroduced. For example, element 230 is first introduced and discussedin reference to FIG. 2. In addition, any dimensions, angles and otherspecifications shown in the figures are merely illustrative ofparticular embodiments of the invention. Accordingly, other embodimentsof the invention can have other dimensions, angles and specificationswithout departing from the spirit or scope of the present invention.

FIG. 2 is a cross-sectional view of a portion of a composite panelassembly 200 having an internally stiffened composite panel 201configured in accordance with an embodiment of the invention. In oneaspect of this embodiment, the composite panel 201 includes a first facesheet 202, a second face sheet 204 offset from the first face sheet 202,and a stiffener 230 disposed between the first and second face sheets202 and 204. Core portions 206 (shown as a core portion 206 a and a coreportion 206 b) can be sandwiched between the first and second facesheets 202 and 204 on opposite sides of the stiffener 230. Each of thecore portions 206 can have a first core face 207 and a second core face208 at least partially bonded to the first and second face sheets 202and 204, respectively. Further, each of the core portions 206 can beconfigured to fit snugly against the stiffener 230 for bonding to thestiffener 230.

In another aspect of this embodiment, the stiffener 230 includes a firstattachment portion, such as a first flange portion 231, a secondattachment portion, such as a cap portion 232, and a third attachmentportion, such as a second flange portion 233. The first and secondflange portions 231 and 233 can be attached, such as by adhesivebonding, to the first face sheet 202, and the cap portion 232 can besimilarly attached to the second face sheet 204. In a further aspect ofthis embodiment, the stiffener 230 also includes an intermediateportion, such as a first web portion 234 and a second web portion 235,extending from the cap portion 232 to the first and second flangeportions 231 and 233, respectively.

In another aspect of this embodiment, the cap portion 232 is configuredto receive and engage a fastener 220 for attaching a part 210 to thecomposite panel 201. In a further aspect of this embodiment, thefastener 220 can be a blind fastener extending only through the part210, the second face sheet 204, and the cap portion 232. In otherembodiments, other types of fasteners, such as a standard fastenerrequiring access to both ends of the fastener for installation (e.g., atypical nut and bolt arrangement), can be used to attach the part 210 tothe composite panel 201. If a standard fastener is used, then accessholes (not shown) can be disposed in the first face sheet 202 betweenthe first and second flange portions 231 and 233 to provide access tothe end of the fastener that would otherwise be inaccessible. In theillustrated embodiment, the part 210 may be a frame or similar memberhaving a U-shaped cross-section. In other embodiments, other partshaving other shapes can be attached to the composite panel 201 ingenerally the same manner as the part 210.

In one embodiment, the first and second face sheets 202 and 204 caninclude an exterior titanium foil laminated together with aNi/Al/graphite layer and a graphite/epoxy layer. In one aspect of thisembodiment, the titanium foil can be positioned on the exterior of thecomposite panel 201 to protect the panel and provide a durable surface.In other embodiments, the first and second face sheets 202 and 204 caninclude other non-metallic materials, such as fiberglass. In furtherembodiments, the first and second face sheets 202 and 204 can includemetallic materials such as aluminum.

In another aspect of this embodiment, the core portions 206 can includea honeycomb material, such as an aramid honeycomb material like the N636product offered by the Dupont Corporation. In other embodiments, thecore portions 206 can include other types of materials, such as aluminumhoneycomb materials or foamed aluminum. In yet other embodiments, thecore portions 206 can include other materials such as phenolic honeycombmaterials and/or non-metallic foam materials. In a further aspect ofthis embodiment, the stiffener 230 can include a graphite/epoxymaterial. In other embodiments, the stiffener 230 can include othermaterials such as metallic materials, including aluminum, titaniumand/or steel.

In one aspect of this embodiment, if the first and second face sheets202 and 204 and the stiffener 230 include an epoxy matrix, then theseparts can be assembled together in an uncured or “green” state, and theassembly can be co-cured to bond the respective parts together. Inanother embodiment, an adhesive such as an epoxy film adhesive can beapplied to the mating surfaces of the respective parts to bond therespective parts together. Heat and/or pressure can be applied to thecomposite panel 201 to facilitate the curing process and form a strongbond-line. A “fly away” tool can be positioned between the first andsecond web portions 234 and 235 of the stiffener 230 to prevent thefirst face sheet 202 from sagging inward during the curing process. Thisfly away tool can be a water-soluble tool that can be easily removedfrom the composite panel 201 after the composite panel 201 has fullycured. In another embodiment, other methods can be used to assemble thecomposite panel 201 without departing from the spirit or scope of thepresent disclosure.

One feature of embodiments of the invention is that the stiffener 230 isbonded or otherwise attached to both the first and second face sheets202 and 204. An advantage of this feature is that out-of-plane loadsintroduced from the part 210 (i.e., loads normal to the second facesheet 204) are effectively distributed over a large portion of thecomposite panel 201 instead of being reacted by a small region of thesecond face sheet 204 which can lead to pull-through of the fastener220. Another feature of embodiments of the invention is that thefastener 220 does not extend completely through the composite panel 201.An advantage of this features is that it reduces or eliminates thelikelihood for rain or other fluids to enter the composite panel 201 orcompromise the integrity of the core or bond lines within the compositepanel 201. Yet another feature of embodiments of the invention is thatthe cap portion 232 can accommodate a range of positions for thefastener 220. An advantage of this feature is that the fastener 220, andhence the part 210, does not have to be precisely located during theattachment to the composite panel 201.

FIGS. 3A-D are partially exploded isometric views illustrating amanufacturing sequence for the composite panel 201 of FIG. 2 performedin accordance with an embodiment of the invention. In one aspect of thisembodiment; the manufacturing sequence that follows assumes that someparts of the composite panel 201 (such as the first and second facesheets 202 and 204, and the stiffeners 230) include composite materialssuch as graphite/epoxy materials. In other embodiments, the same or agenerally similar sequence can be used for other face sheet materialssuch as metallic materials. In further embodiments, other manufacturingsequences can be used to assemble the composite panel 201.

Referring to FIG. 3A, in another aspect of this embodiment, the firstface sheet 202 is positioned on a tool such as a form 360 in the uncuredstate (i.e., a “green” state) so that it will assume the shape of theform 360. For example, in one embodiment, the form 360 can be curved togive the composite panel 201 (FIG. 2) an arcuate shape when finished. Inother embodiments, the form 360 can be flat or have other shapes,depending on the desired final shape of the composite panel 201.

In a further aspect of this embodiment, after the first face sheet 202has been positioned on the form 360, one or more of the stiffeners 230are positioned on the first face sheet 202. For example, in oneembodiment, the stiffeners 230 can be at least generally straightmembers positioned on the first face sheet 202 in a spaced-apartparallel configuration. The spacing between the stiffeners 230 can bedictated by the structural requirements of the finished panel. In oneaspect of this embodiment, the stiffeners 230 can be uncured, such thatsubsequent co-curing of the first face sheet 202 and the stiffeners 230causes the epoxy matrix of the stiffeners 230 to bond to the epoxymatrix of the first face sheet 202. In other embodiments, an adhesivesuch as an epoxy adhesive can be applied to the interface between thestiffeners 230 and the first face sheet 202 to bond the stiffeners 230to the first face sheet 202 during the curing process.

Referring next to FIG. 3B, in one aspect of this embodiment, arcuatesecond stiffeners 331 are positioned on the first face sheet 202adjacent to the first stiffeners 230 to give the composite panel 201additional strength and stiffness. In other embodiments, the secondstiffeners 331 can be omitted when not required for structural reasons.In further embodiments, the first stiffeners 230 can be omitted and thesecond stiffeners 331 can be used exclusively. Such an embodiment may beadvantageous in those applications where the longitudinal loads on thepanel are minimal.

Each of the second stiffeners 331 can have a first end 333 and a secondend 335 opposite the first end 333. At least one of the first and secondends 333, 335 can abut one of the first stiffeners 230 to form a gridpattern of stiffeners on the first face sheet 202. For example, inanother aspect of this embodiment, the second stiffeners 331 can bepositioned at an angle 337 relative to the first stiffeners 230. In oneembodiment, the angle 337 can be from about 85° to about 95°. In otherembodiments, the second stiffeners 331 can be positioned at otherangles, such as about 45°, relative to the first stiffeners 230. In afurther aspect of this embodiment, the second stiffeners 331 can have across-sectional shape at least generally similar to the cross-sectionalshape of the first stiffeners 230. In other embodiments, the secondstiffeners 331 can have other cross-sectional shapes. For example, thesecond stiffeners 331 can have simpler cross-sectional shapes if theyare not used for part attachment or if they are not highly loaded.Examples of cross-sectional shapes for both the first and secondstiffeners 230 and 331 in accordance with other aspects of the inventionare described below with reference to FIGS. 5A-G. In yet otherembodiments, one or more of the second stiffeners 331 can be omittedfrom the composite panel 201, depending on factors such as appliedloads.

Referring next to FIG. 3C, in one aspect of this embodiment, a pluralityof the core portions 206 are positioned on the first face sheet 202between the first and second stiffeners 230 and 331. In another aspectof this embodiment, adhesive layers 340 can be applied to the first facesheet 202 before positioning the core portions 206 on the first facesheet 202 for bonding the core portions 206 to the first face sheet 202during the co-curing cycle. The adhesive layer 340 can include a filmadhesive such as an epoxy film adhesive. In other embodiments, othertypes of adhesives can be used to bond the core portions 206 to thefirst face sheet 202. In further embodiments, the adhesive layers 340can be omitted and the epoxy present in the face sheet 202 can bond thecore portions 206 to the face sheet 202 during the co-curing cycle. In afurther aspect of this embodiment, additional adhesive (not shown) canbe applied to those portions of the first and second stiffeners 230 and331 which contact edge regions of the core portions 206 to bond the edgeregions of the core portions 206 to the first and second stiffeners 230and 331.

Referring now to FIG. 3D, in one aspect of this embodiment, the secondface sheet 204 is positioned over the core portions 206 and the firstand second stiffeners 230 and 331 to complete the composite panel 201.Adhesive layers 341 at least generally similar to the adhesive layers340 shown in FIG. 3C can be applied to at least the second core faces208 for bonding the second face sheet 204 to the core portions 206during the co-curing cycle. In other embodiments, the adhesive layers341 can be omitted and the epoxy of the second face sheet 204 can beused to bond the second face sheet 204 to the core portions 206 duringthe co-curing cycle.

In the embodiment described above, the uncured elements of the compositepanel 201 (i.e., the first and second face sheets 202 and 204, and thefirst and second stiffeners 230 and 331) can be co-cured to a hardenedand usable form by applying uniform pressure to the second face sheet204 to compress the elements against the form 360 while simultaneouslyproviding heat to the elements. For example, in one embodiment, thisheat and pressure can be applied by positioning a vacuum bag (not shown)over the composite panel 201, drawing a vacuum, and placing the assemblyin an autoclave or oven. In other embodiments, other methods can beemployed to apply the heat and/or pressure required to co-cure thecomposite panel 201 in a given shape.

As those of ordinary skill in the relevant art will understand, in otherembodiments, other methods and/or other manufacturing equipment can beused to manufacture the composite panel 201 without departing from thespirit or scope of the present invention. For example, in one otherembodiment, the composite panel 201 can be assembled using a toolingmandrel. In this embodiment, the second face sheet 204 can be positionedagainst the mandrel, and the other elements of the composite panel 201(i.e., the first and second stiffeners 230 and 331, the core portions206, and the first face sheet 202) can be assembled on top of the secondface sheet 204. In one aspect of this embodiment, the tooling mandrelcan be a rotating cylindrical mandrel, and the first and second facesheets 202 and 204 can be applied using a moving fiber-placement deviceto wind pre-impregnated graphite/epoxy strands over the mandrel and/orthe respective elements of the composite panel 201.

FIG. 4 is an enlarged cut-away isometric view of a portion of thecomposite panel 201 of FIG. 3D configured in accordance with anembodiment of the invention. For purposes of illustration, the compositepanel 201 is shown fully assembled with parts of the second face sheet204 and the core portions 206 cut away to show how the second stiffener331 joins the first stiffener 230. In one aspect of this embodiment, thefirst end 333 of the second stiffener 331 is configured to at leastpartially abut the first stiffener 230 and is bonded to the firststiffener 230 with a suitable adhesive such as an epoxy adhesive. Inaddition, any gaps between the first end 333 of the second stiffener 331and the first stiffener 230 can be filled, for example, by a plug membersuch as a plug member 440, to prevent the second face sheet 204 fromsagging into these gaps during the co-curing cycle. In otherembodiments, other methods can be used to join the second stiffener 331to the first stiffener 230. For example, in another embodiment, thefirst and second stiffeners 230 and 331 can be preassembled in alattice-type structure before they are positioned on the first facesheet 202. In one aspect of this embodiment, the first and secondstiffeners 230 and 331 can be joined together by fasteners or othermedia during the preassembly. In yet other embodiments, such as lightlyloaded embodiments, the second stiffener 331 can be separate from thefirst stiffener 230, or the second stiffener 331 can be omitted.

In a further aspect of this embodiment, the second face sheet 204 has anexterior surface 403 defining a first direction 401 and a seconddirection 402. The first direction 401 can be parallel to the surface403, and the second direction 402 can be perpendicular to the surface403. In another aspect of this embodiment, the first and second facesheets 204 and 202 can provide structural strength parallel to the firstdirection 401, while the core portions 206 can offset the first facesheet 202 from the second face sheet 204 in the second direction 402.Accordingly, the foregoing arrangement of face sheet—core—face sheet canprovide a relatively strong and lightweight structural panel. Theaddition of the first and second stiffeners 230 and 231 can furtherenhance the structural capability of the composite panel 201.

FIGS. 5A-G are cross-sectional views of composite panel assemblies 500a-g having composite panels 501 a-g configured in accordance with otherembodiments of the invention. Referring to FIG. 5A, in one embodiment,the composite panel 50 a includes a first face sheet 502 a, a secondface sheet 504 a offset from the first face sheet 502 a, and a coreportion 506 a sandwiched between the first and second face sheets 502 aand 504 a. A channel 550 can be machined or otherwise formed in the coreportion 506 a to make room for a low-profile stiffener 530 a. In oneaspect of this embodiment, the stiffener 530 a can be generally similarto the stiffener 230 of FIG. 2. However, the stiffener 530 a has a lowerprofile and is fixedly attached, such as by bonding, to only the secondface sheet 504 a. Accordingly, the fastener 220 can attach the part 210to the composite panel 501 a in a manner at least generally similar tothe manner described above with reference to FIG. 2. Although thecomposite panel 501 a may not be as strong as the composite panel 201described above with reference to FIG. 2, the composite panel 501 a maybe desirable in lightly loaded structures where panel weight is afactor.

Referring next to FIG. 5B, in one embodiment, the composite panel 501 bincludes a Z-section stiffener 530 b having a first attachment portionsuch as a first flange portion 531 b attached to a first face sheet 502b, and a second attachment portion such as a second flange portion 532 battached to a second face sheet 504 b. An intermediate portion such as aweb portion 534 b can extend between the first and second flangeportions 531 b and 532 b. Accordingly, the fastener 220 can fixedlyattach the part 210 to the composite panel 501 b in a manner at leastgenerally similar to the manner described above with reference to FIG.2.

Referring next to FIG. 5C, in one embodiment, the composite panel 501 cincludes a closed-section stiffener 530 c having a first attachmentportion such as a first flange portion 531 c attached to a first facesheet 502 c, and a second attachment portion such as a second flangeportion 532 c attached to a second face sheet 504 c. An intermediateportion such as a first web portion 534 c, and a second web portion 535c offset from the first web portion 534 c, extends between the first andsecond flange portions 531 c and 532 c, defining an enclosedcross-sectional shape. Accordingly, the fastener 220 can fixedly attachthe part 210 to the composite panel 501 c in a manner at least generallysimilar to the manner described above with reference to FIG. 2.

Referring now to FIGS. 5D-G, collectively, these figures illustrateembodiments of composite panels 501 d-g having stiffeners 530 d-g,respectively, that are variations of the stiffeners described above. Forexample, in FIG. 5D the stiffener 530 d is similar to the stiffener 530c of FIG. 5C, except that the first flange portion 531 c is omitted.Similarly, referring to FIG. 5E, the stiffener 530 e is similar to thestiffener 530 d, except that the stiffener 530 e has a lower profile,similar to the stiffener 530 a of FIG. 5A. Further, referring to FIG.5F, the stiffener 530 f is similar to the stiffener 530 d of FIG. 5D,except that a web portion 535 d is omitted. Referring to FIG. 5G, thestiffener 530 g is similar to the stiffener 530 c of FIG. 5 c, exceptthat the first web portion 534 c is omitted.

One feature of embodiments of the invention as illustrated above inFIGS. 5A-G is that the stiffeners can have a wide array ofconfigurations. An advantage of this feature is that a particularstiffener can be selected based on the structural requirements of thepanel into which it is incorporated. For example, for lightly loadedpanels where weight is a factor, a stiffener such as 530 e or 530 f maybe an appropriate selection. For highly loaded panels, a stiffener suchas 530 b, 530 c or 530 g may be more appropriate.

Another feature of embodiments of the invention is that in selectedconfigurations, the stiffener can provide a conduit or enclosedpassageway through the respective composite panel. For example, both thestiffener 530 c of FIG. 5C and the stiffener 230 shown in FIG. 2 provideat least generally closed passageways extending through their respectivecomposite panels. One advantage of this feature is that thesepassageways can be used as conduits for various systems such aselectrical wiring and/or control cables, or liquids such as hydraulicfluid or fuel, depending on the particular application.

FIG. 6 is an isometric view of an aircraft 650 having composite panels601 a-c configured in accordance with embodiments of the invention. Inone aspect of this embodiment, the aircraft 650 includes a fuselage 652and a wing 654 fixedly attached to the fuselage 652. In another aspectof this embodiment, the fuselage 652 includes a composite panel 601 a,and the wing 654 includes a composite panel 601 b. The composite panels601 a, b can be at least generally similar in construction as thecomposite panel 201 described above with reference to FIGS. 2-3D. In afurther aspect of this embodiment, a control surface, such as anelevator 656, can also include a composite panel 601 c that is at leastgenerally similar to the composite panel 201. In any of the compositepanels 601 a-c, fasteners can extend through one or both of the firstand second stiffeners 230 and 331 to attach the respective panel to anadjacent internal airframe structure. In other embodiments, compositepanels at least generally similar to the composite panel 201 can be usedas internal airframe structural members.

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from thespirit and scope of the invention. For example, although embodiments ofthe invention have been described above with reference to compositepanels employing nonmetallic face sheets, in other embodiments, therespective face sheets can be made from metallic materials such asaluminum or titanium. In addition, while various embodiments of theinvention may be well-suited for aerospace applications such asaircraft, in other embodiments, composite panels in accordance with theinvention can be used in the construction of other structures such asmarine and land vehicle structures or fixed structures. Accordingly, theinvention is not limited, except as by the appended claims.

1-37. (canceled)
 38. An aircraft comprising: a fuselage; and a wingfixedly attached to the fuselage, wherein at least one of the fuselageand the wing includes a composite panel, the composite panel including:a first face sheet; a second face sheet offset from the first facesheet; at least one core portion extending between the first and secondface sheets; and a stiffener disposed between the first and second facesheets at least proximate to the at least one core portion, thestiffener having at least one attachment portion attached to one of thefirst and second face sheets.
 39. The aircraft of claim 38 wherein theat least one attachment portion of the stiffener includes a first flangeportion attached to the first face sheet, and wherein the stiffenerfurther includes a cap portion and a second flange portion, the capportion being offset from the first flange portion and attached to thesecond face sheet, the second flange portion being offset from the capportion and attached to the first face sheet.
 40. The aircraft of claim38 wherein the composite panel is an external fuselage skin panelfixedly attached to the fuselage, wherein the first face sheet is afirst arcuate panel, the second face sheet is a second arcuate panel,and the stiffener is an elongate member extending between the first andsecond face sheets.
 41. The aircraft of claim 38 wherein the at leastone attachment portion of the stiffener is configured to receive andengage a fastener to fixedly attach the composite panel to the at leastone of the fuselage and the wing.
 42. The aircraft of claim 38 whereinthe composite panel is an external fuselage skin panel fixedly attachedto the fuselage, wherein the at least one core portion includes ahoneycomb material, and wherein the first and second face sheets includea graphite/epoxy material. 43-62. (canceled)
 63. The aircraft of claim38, further comprising means for attaching the first and second facesheets to the core portion.
 64. The aircraft of claim 38 wherein thefirst face sheet is positioned between the stiffener and a part, andwherein the aircraft further comprises means for attaching the part tothe stiffener.
 65. An aircraft comprising: a fuselage; and a wingfixedly attached to the fuselage, wherein at least one of the fuselageand the wing includes a composite panel, the composite panel including:a first face sheet offset from a second face sheet; a stiffener having afirst attachment portion attached to the first face sheet, a secondattachment portion attached to the second face sheet, and anintermediate portion extending between the first and second attachmentportions; a first core portion positioned between the first and secondface sheets on a first side of the stiffener, wherein at least a firstportion of the stiffener contacts the first core portion; and a secondcore portion positioned between the first and second face sheets on asecond side of the stiffener opposite to the first side of thestiffener, wherein at least a second portion of the stiffener contactsthe second core portion, and wherein the stiffener defines an openpassageway extending between the first and second core portions.
 66. Thecomposite panel of claim 65 wherein the stiffener further includes athird attachment portion attached to the first face sheet, the thirdattachment portion being offset from the second attachment portion andspaced apart from the first attachment portion.
 67. The composite panelof claim 65 wherein the first face sheet is a first arcuate panel, thesecond face sheet is a second arcuate panel, and the stiffener is anelongate straight member extending between the first and second facesheets.
 68. The composite panel of claim 65 wherein the secondattachment portion is configured to receive and engage a fastener tofixedly attach a part to the composite panel.
 69. A method formanufacturing a composite panel, the method comprising: providing afirst face sheet; providing a second face sheet; providing a coreportion, the core portion having a first core face and a second coreface opposite the first core face; providing a stiffener having a firstattachment portion, a second attachment portion and an intermediateportion, the second attachment portion being offset from the firstattachment portion, and the intermediate portion extending between thefirst and second attachment portions; positioning the first attachmentportion of the stiffener at least proximate to the first face sheet;positioning the first core face of the core portion adjacent to thefirst face sheet and at least proximate to the stiffener; andpositioning the second face sheet at least proximate to the second coreface and the second attachment portion of the stiffener.
 70. The methodof claim 69 wherein providing a core portion includes providing a firstcore portion, and wherein the method further comprises: positioning thefirst core portion toward one side of the stiffener between the firstand second face sheets; and positioning a second core portion toward asecond side of the stiffener between the first and second facesheets,wherein the stiffener defines an open passageway extending between thefirst and second core portions.
 71. The method of claim 69, furthercomprising: positioning a part at least proximate to the second facesheet adjacent to the second attachment portion of the stiffener; andinserting at least one fastener through the part, the second face sheet,and the second attachment portion of the stiffener to attach the part tothe stiffener.
 72. The method of claim 71 wherein inserting at least onefastener includes installing a blind fastener through the part, thesecond face sheet, and the second attachment portion of the stiffener.73. The method of claim 69 wherein the stiffener is a first stiffener,and wherein the method further comprises: providing a second stiffenerhaving at least a third attachment portion; positioning the thirdattachment portion of the second stiffener at least proximate to thesecond face sheet; positioning a part at least proximate to the secondface sheet adjacent to the second attachment portion of the firststiffener and the third attachment portion of the second stiffener;inserting a first fastener through the part, the second face sheet, andthe second attachment portion of the first stiffener to attach the partto the first stiffener; and inserting a second fastener through thepart, the second face sheet, and the third attachment portion of thesecond stiffener to attach the part to the second stiffener.
 74. Themethod of claim 69 wherein the method further comprises: positioning afirst adhesive layer between the first core face and the first facesheet; and positioning a second adhesive layer between the second coreface and the second face sheet.
 75. The method of claim 74, furthercomprising: applying pressure to one or both of the first and secondface sheets; and heating one or both of the first and second face sheetsto bond one or both of the first and second face sheets to the coreportion and the stiffener.
 76. The method of claim 69 wherein thestiffener is a first stiffener, and wherein the method further comprisespositioning a third attachment portion of a second stiffener at leastproximate to the first face sheet and at an angle relative to the firststiffener, wherein positioning the second face sheet at least proximateto the second core face and the second attachment portion of the firststiffener includes positioning the second face sheet at least proximateto the second stiffener.
 77. The method of claim 69 wherein thestiffener is a first stiffener, and wherein the method furthercomprises: positioning a third attachment portion of a second stiffenerat least proximate to the first face sheet and at an angle relative tothe first stiffener, wherein the second stiffener has a first endportion in a first direction and a second end portion in a seconddirection opposite to the first direction; and attaching the first endportion of the second stiffener to the first stiffener, whereinpositioning the second face sheet at least proximate to the second coreface and the second attachment portion of the first stiffener includespositioning the second face sheet at least proximate to the secondstiffener.